1. Field of the Invention
This invention relates generally to thin films, and particularly to thin film compositions and fabrication methods which yield films with high resistivity and a low temperature coefficient of resistance.
2. Description of the Related Art
Integrated circuit (IC) resistors are typically formed from a thin film (TF) material which is deposited on a substrate and formed into features having desired sizes and shapes as needed to provide respective resistances.
Thin films have several characteristics that affect their suitabilty for a particular application. A film's sheet resistance (Rs) and resistivity (ρ) determine how much resistance a particular TF feature can provide, while its temperature coefficient of resistance (TCR) describes how the feature's resistance varies with temperature. An ideal TF will have high sheet resistance and resistivity characteristics and a low TCR, thereby minimizing the die area they require and providing a resistance which is stable over temperature.
Conventional TF resistors are made from a composition comprising silicon and chromium (SiCr). Though generally adequate, these resistors have limitations that may make them unsuitable for some applications. For example, battery-powered devices require power consumption to be as low as possible. As current through a resistor is inversely proportional to its value, such applications often require high resistance resistors. However, conventional TF resistors typically have a sheet resistance of 2 kΩ/□ or less, and thus can require an unacceptably large die area to provide a desired resistance value.
In addition, conventional thin films typically have a thickness of about 100 . This can result in conduction currents in the TF feature being concentrated near the surface of the material, which can degrade the feature's reliability.
One approach that improves upon conventional thin films is disclosed in U.S. Pat. No. 6,217,722 to Jankowski et al. The films described there comprise titanium, chromium, aluminum and oxygen (Ti—Cr—Al—O), which are said to be capable of providing resistivity values of 104 to 1010 ohm-cm. However, the described method requires the use of two component gasses (argon and oxygen), and makes no assertions with respect to the TCR of the resulting resistors.
Another approach to thin film resistor fabrication is described in U.S. Pat. No. 6,129,742 to Wu et al. Here, the resulting resistors may possess a relatively low TCR, but only for thin films having a relatively low sheet resistance; higher sheet resistances result in a TCR value which may be unacceptably high.